Conventional heat transfer label assemblies typically comprise a carrier on which the label is supported, for example, paper. A release layer or coating, for example, a wax, overlies the carrier. If needed, a protective layer may overlie the release layer to provide protection to the ink when transferred to a container. An adhesive layer or coating may overlie the ink for adhering the ink to the container. The label is transferred to a container by applying heat and pressure to the heat transfer label assembly. The heat softens the release layer and allows the ink (along with the optional primer and/or adhesive) to separate from the carrier, while the application of pressure transfers the ink to the container. The carrier is typically discarded.
In a conventional heat transfer label with a wax release layer, the wax is applied at elevated temperatures as a 100 wt % solids coating (i.e., as a molten wax) in an amount of about 6 to about 8 lb/ream. Unfortunately, this high temperature, high solids, high coat weight application often requires costly specialized equipment. Additionally, the coating process typically needs to be conducted offline to allow for sufficient time for the wax to cool and solidify before being able to print or otherwise apply the remaining layers of the heat transfer label assembly.
To overcome these obstacles, the present inventors developed a release layer composition including a lower solids content that could be applied at lower coat weights at ambient temperature. The composition generally included a solvent and one or more solids for forming the release layer. To prevent the solvent and/or solids from penetrating the paper and, therefore, leaving too little solids on the surface of the paper, the present inventors used a substantially solvent impermeable paper as the carrier, for example, a paper with an electron beam (EB) crosslinked polymer coating on the surface. Since substantially all of the release layer composition was retained on the surface of the polymer coated paper, the weight of the dry release layer (e.g., less than 4 dry lb/ream) was able to be markedly reduced while still providing the desired release characteristics (as compared with a conventional wax release layer).
Despite the success of this heat transfer label assembly, the present inventors recognized that the use of such substantially solvent impermeable papers may be cost prohibitive for some applications. Further, the present inventors observed that the low solids release layer composition was sometimes prone to underdrying. In such instances, when the heat transfer label assembly was wound into a roll, the retained solvent in the release layer would soften the adjacent layers of the heat transfer label assembly slightly, which in turn, led to blocking of the roll (i.e., one side of the heat transfer label assembly sticking to the other side of the heat transfer label assembly) and, therefore, problems with releasing the label during application of the heat transfer label to a container.
Thus, there remains a need for a release layer composition that can be applied at ambient temperature in line with the heat transfer label assembly manufacturing process, where the resulting release layer provides the desired release characteristics for the heat transfer label assembly without blocking. There is also a continuing need for a reduced cost heat transfer label assembly.